The present invention concerns a traveling wave tube comprising a sleeve cut with grooves. It also relates to a process for manufacturing the sleeve cut with grooves.
The invention concerns the field of traveling wave tubes having a helix-type delay line, i.e. for example, a simple helix delay line, or a "ring and bar" or "ring and loop" delay line.
However, in order to simplify the present description, the "delay line" referred to herein-below will be understood to be a simple helix.
The helix delay line is placed in a cylindrical sleeve, which is generally metallic, to which it is fixed through the intermediary of dielectric supports.
For traveling wave tubes operating at relatively low power levels, the helix and the supports are assembled by clamping in the sleeve. The helix is made of tungsten, for example, and the supports are made for example of quartz, alumina, glucina or boron nitride. The sleeve can be made of copper or stainless steel.
For traveling wave tubes operating at high power levels, the helix is brazed to the dielectric supports that are brazed to the sleeve. The helix can thus be made of copper, like the sleeve, and the dielectric supports can be made for example of beryllium oxide.
Three dielectric supports are generally used that are regularly disposed at 120.degree. from one another.
In traveling wave tubes, in particular, in wide-band tubes operating at a high power level, noise or interference oscillations are produced at frequencies where the dephasing of the hyper-frequency wave transmitted is close to .pi. between two consecutive turns of the helix.
In order to avoid these oscillations, it is known to cause to vary the length of the turns of the helix along the axis of the tube and, correlatively, the pitch of the helix in order to maintain the synchronism conditions in the operating frequency band.
Helixes, called conical helixes, wound on a conical mandrel and the pitch of which increases in the same ratio as the diameter have been realized.
The practical realization of these conical helixes is difficult, above all when the helix, dielectric supports and sleeve are brazed.
When the focussing of the tube is realized by permanent magnets periodically alternated at frequent intervals, which is often the case, it is preferable to dispose of an externally cylindrical sleeve in order to support the annular magnets and the currently manufactured pole pieces. A sleeve can thus be used which is internally and externally cylindrical and dielectric supports having an appropriate outline, i.e. comprising a cylindrical surface in contact with the sleeve and a conical surface in contact with the helix. The drawback of this solution is that these very particular supports are expensive and difficult to realize.
In order to overcome these drawbacks, the applicant proposed in French Pat. No. 76.28319, published under No. 365 218, a pseudo-conical helix presenting flat parts parallel to the axis of the tube and at a constant distance from it. This solution allows the utilization of a cylindrical sleeve and dielectric supports of constant height.
The drawback of pseudo-conical helixes is that when the hyper-frequency power increases, noise or interference oscillations can be produced, close to mode .pi., towards the high frequency output of the tube there where the high frequency field is the highest, while for these power levels, no oscillations are produced if truly conical helixes are used. Indeed, conical helixes present the advantage of combining a variation of the length of the turns along the axis of the tube with a decrease of coupling impedance the more important the frequency is higher. This has the effect of sharply reducing the energy transfer to close to mode .pi. and due to this of suppressing oscillations. It should be noted that calculations and experiences have shown that, contrary to what was admitted, the coupling impedance between the hyper-frequency field and the electrons beam does not have to be maximal along all the length of the tube and that, in particular, the interaction ouput can be improved with a coupling impedance decreasing towards the end of the line, there where the HF field is maximum.
The present invention allows to overcome the problem that consists in manufacturing a traveling wave tube with a conical helix, currently manufactured dielectric supports of a constant height and an externally cylindrical sleeve.